GB894593A - Improvements in or relating to shaped bodies composed of porous material, such as graphite or carbon, and having a gas tight and liquid tight surface - Google Patents

Abstract

A body consisting of carbon, graphite or silicon carbide is coated with a first layer comprising at least one of the carbides of Ti, Cr, Mo, Si and Zr with a second layer consisting of at least one of the elements Mo, Cr, Ti, Zr, Si D/or combinations of these with one of the elements N2, Bl, B, Al, Si, P or C. The layers may be applied as suspensions of reactive powders in dextrin or polyvinyl alcohol or as a paste and the coating is then dried and heated to form the required coating composition. In an example a coating of TiC and SiC is formed by applying a suspension of TiSi2 and TiC in polyvinyl alcohol to a graphite body and heating to 1600 DEG C. in an inert atmosphere. A suspension of CrSi2 and SiC in dextrin applied to graphite and heated to 1600 DEG C. in an inert atmosphere forms a coating of Cr3C2 and SiC and in another example a coating of chromium is formed on carbon by applying a suspension of chromium powder in dextrin and by heating to 1900 DEG C. in argon an intermediate layer of Cr3C2 is formed. In a further example a suspension of Cr3Si, Ti and ethyl silicate in alcohol is sprayed on to a carbon body and the coating is then fused. Other methods such as flame spraying and dipping (see Group II) are described.ALSO:A body consisting of carbon, graphite or silicon carbide is coated with a first layer comprising at least one of the carbides of Ti, Cr, Mo, Si and Zr, and with a second layer consisting of at least one of the elements Mo, Cr, Ti, Zr, Si and/or combinations of these with one of the elements N2, Be, B, Al, Si, P or C. The first layer may also contain combinations of the elements Mo, Cr, Ti, Zr and Si with the elements N2, Be, B, Al, Si or P e.g. Ti Si2, Ti N, Cr3 Si. The first carbide layer may be applied directly as a carbide or alternately the metal may be applied to the body and the carbide formed by reaction between the metal and the carbon of the body. The two layers are preferably formed by a two step process although both may also be applied simultaneously. The first coating layer may be applied by any of the following methods:-(1) As a suspension of the reactive powders in dextrin or alcohol followed by heating to form the required coating e.g. a mixture of Ti Si2 and Ti C in polyvinyl alcohol which on heating forms a layer of Ti C and Si C. (2) by flame spraying e.g. Ti wire is flame sprayed in acetylene then heated at 800 DEG C.-1000 DEG C. in nitrogen to form a first layer of Ti C and an outer layer of nitride, or heated at 1600 DEG C. in argon to produce a Ti C layer. Similarly Zr is flame sprayed in oxyhydrogen or acetylene and then heated in carbon monoxide at 2500 DEG C. to form Zr. C. (3) Immersion in a fused bath of the coating material e.g. in fused Ti Si2 under argon. The outer covering layer is also applied by any of the above methods or by heating a stoichiometric mixture of the constituents to reaction temperature e.g. a mixture of titanium hydride and silicon heated at 1350 DEG C. to form a Ti Si2 outer layer, or by reaction of a sprayed metal surface with a gaseous halide e.g. Zr heated to 1100 DEG C. in the presence of a gaseous atmosphere of Si Cl4 and H2 to form a coating of Ti Si2. The coated body is finally heated to fuse the outer layer. Examples are given of the formation of the following coatings on carbon or graphite:- (1) Ti C, Si C with an outer layer of Ti Si2. (2) Ti C with an outer layer of Ti N. (3) Cr3 C2 with an outer layer of Cr. (4) Ti C, Cr3 Si with an outer layer of Ti. (5) Zr C with an outer layer of Zr Si2. (6) Ti C, Si C with an outer layer of Ti Si2. (7) Cr3 C2, Si C with an outer layer of Ti Si2, Mo Si. Silicon carbide is also coated with an intermediate Ti C layer and an outer cover of Zr Si2.